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A speculation

Submitted by Kevin Jardine on 18 March, 2006 - 23:58

Our Milky Way galaxy began forming shortly after the universe was created some 13 billion years ago. Much of its dust and gas were drawn from smaller galaxies, globular clusters, and molecular clouds in the region. Each rotation of the giant whirlpool saw more matter sucked by gravity into its interior. Smaller galaxies including the Large Magellanic Cloud and the Small Magellanic Cloud as well as the Sagittarius Dwarf continue to be torn apart and drawn into the Milky Way.

Molecular clouds also continue to be attracted by the Milky Way's gravity. Between 30 and 60 million years ago one giant molecular cloud about 400 parsecs long crashed through the disk of the Milky Way, about two thirds towards the visible edge of the galaxy. As the disk is not solid, the cloud passed right through, but galactic gravity pulled it back up again. After several oscillations, the cloud, which had previously been in orbit around the galaxy, was gradually absorbed into the disk. Not entirely, however - the material remained tilted at an odd angle of about 17 degrees, revealing its external origins.

The shockwaves caused by the cloud's collision with the disk disturbed the dust and gas within, causing some of it to condense. Out of these dust cocoons were born huge, hot O and B class stars that formed the Cassiopeia-Taurus star association. Over the following millions of years, stellar winds and exploding supernovae from the stars in this association created bubbles of hot expanding gas that formed froth in the molecular cloud much like soap bubbles in a bubble bath or kitchen sink. Gradually some of these bubbles grew so large that they burst through the top and bottom of the thin galactic disk, creating a cosmic "chimney". The pressure from bubbles surrounding the Cassiopeia-Taurus OB association continued through the disk as well, creating an expanding ring of dust and gas. Eventually this ring struck more solid material.

Towards the eastern outer edge of the ring, this more solid material was the thick edge of another expanding bubble from the Vela region. The collision of the two expanding bubbles created a new generation of stars now called Orion the Hunter. One by one, the great stars of Orion's belt, Mintaka, Alnilam and Alnitak, winked into life, as did Theta-1 Orionis and the vast star cluster called the Trapezium at the heart of the Orion nebula.

Towards the western inner edge of this ring, this more solid material was a series of dense dust clouds called the Rift. This collision occurred in the vicinity of an old red supergiant called Antares and the bright new stars that soon joined Antares are now called Scorpius, the Scorpion. Other less famous star groupings formed along the expanding and tilted ring: Lupus the Wolf, Ophiuchus the Serpent-Bearer, and Cepheus OB6.

By now the once mighty Cassiopeia-Taurus association was a spent force, drifting and disintegrating into smaller clusters. Only one brilliant star surrounded by a small cluster remained to mark the heart of the association: the yellow supergiant Mirfak, Alpha Perseus.

The child star associations began to make their marks on the Milky Way. New bubbles grew around Orion and Scorpius. These associations were too young yet to form supernovae, but stellar winds from their brightest stars were enough to create immense turbulence. Eventually gas from one of the bubbles surrounding the Scorpius association broke through the walls of a smaller bubble created in the distant past. Hot gas poured into an area occupied by an old and dim former star cluster called the Ursa Major Moving Group. Former star cluster because now, 500 million years after its formation, the cluster was no longer held together by gravity, but could still be distinguished by its common motion.

At the edge of the Ursa Major Moving Group was a far older G2 main sequence star. It had formed some 4.5 billion years ago in the inner galaxy - almost half the age of the galaxy itself. It was just passing by the Ursa Major and Scorpius stars. At this point, far from its origin, the raging gas from the Scorpius association had cooled off and slowed down. Nevertheless, it buffetted the heliopause of the G2 star (the place where its stellar wind struck the interstellar gas). Some radiation made it through the heliopause. It passed the belt of icy outer planets, the middle belt of gas giants, and then reached the inner region where four rocky planets orbited: grey, white, blue and red. The blue planet had an unusually strong magnetic field. The interaction with the Scorpius radiation caused its upper atmosphere to glow.

Down on the surface, an ape-like creature walked out of a cave, looked up, and saw the bright green and pink curtains shimmering before the stars.